The invention relates to a microlithographic projection exposure apparatus of the kind used for manufacturing large-scale integrated electrical circuits and other microstructured components. The invention further relates to a projection lens of such a microlithographic projection exposure apparatus and to a process for the microlithographic manufacture of microstructured components.
Projection exposure apparatuses project structures contained in a reticle onto a photosensitive layer which may be applied, for example, to a silicon wafer. One of the fundamental objectives in developing such projection exposure apparatuses is to be able to lithographically define structures of increasingly small dimensions on the photosensitive layer. Small structures give rise to high integration densities, which have a generally favorable effect on the efficiency of the microstructured components produced by means of such apparatuses. The size of the definable structures depends above all on the resolution of the projection lens used. Because the resolution of the projection lens is inversely proportional to the wavelength of the projection light, one approach to increasing resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths currently used are in the ultraviolet spectral range and are of 193 nm and 157 nm. Another approach to increasing resolution is based on the concept of introducing an immersion liquid into the space intervening between the last lens of the projection lens on the image side and the photosensitive layer to be exposed. The immersion liquid preferably has the same refractive index as the photosensitive layer, whereby the numerical aperture is enlarged
The projection exposure apparatus known from U.S. Pat. No. 4,346,164 has for this purpose an upwardly open container which holds the carrier with the photosensitive layer applied thereto. The container is filled with an immersion liquid and is so arranged that the last optical element of the projection lens on the image side can be immersed in the liquid.
It has been shown, however, that although resolution can be increased with the aid of the immersion method the contrast of the structures imaged on the photosensitive layer is often unsatisfactory.